Internal-Friction and Young's-Modulus Variations in the Superconducting, Mixed, and Normal States of Niobium
Internal-friction and Young's-modulus measurements have been performed on as-grown and deformed niobium single crystals as a function of temperature and external magnetic field. The crystals are driven in longitudinal resonance between 1.5 and 10°K at 80 and 240 kHz. An internal-friction relaxation peak is observed at 3.24°K in the superconducting state, but shifts abruptly to 2.08°K when the mixed state is established. This peak is characterized by a single relaxation time τ, defined by the expression τ=τ0exp(EkT). The activation energy E and the attempt frequency τ0-1, respectively, are 0.0019 eV and 6×108 rad/sec in the superconducting state, and 0.0016 eV and 4×109 rad/sec in the normal state (k is Boltzmann's constant and T denotes the absolute temperature). Effects of plastic deformation and residual-impurity content suggest that the relaxation process involves the motion of dislocations, modified by the presence of residual chemical impurities. Based on this premise, the interaction between dislocations and magnetic fluxoids is inferred to be attractive. Young's modulus also reflects the relaxation, and, in addition, decreases with decreasing temperature below 4°K. This decrease, however, is not consistent with ordinary relaxation mechanisms.